The present invention relates to Raman amplified optical systems and, more particularly, to WDM optical systems utilizing Raman amplification techniques.
The subject of Raman amplification is well-known in the literature. Stimulated Raman amplification is a nonlinear optical process in which an intense pump wave is injected into an optical fiber carrying one or more optical message signals. In fused silica fibers, if the pump wavelength is approximately 100 nm shorter than the signal wavelength, the pump will amplify the signal(s) via stimulated Raman scattering. If the amplification is made to occur in the transmission fiber itself, the amplifier is referred to as a xe2x80x9cdistributed amplifierxe2x80x9d. In such a distributed amplifier arrangement the pump can either propagate through the fiber in the same direction as the signal(s) (referred to as a xe2x80x9cco-propagating arrangementxe2x80x9d), or in the opposite direction (referred to as a xe2x80x9ccounter-propagating arrangementxe2x80x9d). In either case, amplification will occur, although there are well-known and discussed differences between the co- and counter-propagating arrangements.
When using Raman amplification in multiwavelength optical communication systems, pump depletion modulation (PDM) becomes problematic. In particular, crosstalk among wavelength division multiplexed (WDM) channels arises and is caused by a two-step process in which extraction of energy from the initial pump wave by a first modulated channel (e.g., channel xe2x80x9cAxe2x80x9d) modulates the pump by depletion, followed by the now-modulated pump in turn modulating a second channel (e.g., channel xe2x80x9cBxe2x80x9d) also being amplified. The pump-mediated crosstalk is of constant amplitude as the modulation frequency increases, but beyond some threshold, decreases to a tolerable level (in accordance with a relationship dependent on the reciprocal of frequency).
Utilizing a counter-propagating pump source has been found to reduce the PDM effect, but as the number of channels in WDM systems increase, additional PDM-based problems arise. In particular, there is a need to increase the power coupled from the Raman amplifier into the fiber channel to support such a demand for increasing the number of channels. However, this is opposed by an increase in non-linearity penalties in the system. Further, the number of WDM channels requires precise monitoring to remotely extract the pump signal. Most difficult, however, is the fact that the polarization state within a Raman amplifier (i.e., usually several kilometers in length) can change as a function of intrinsic and/or extrinsic stresses, temperature and environmental factors.
One solution to the polarization problem is to use a pair of orthogonal pump lasers, where the output of the pair of lasers is combined using a polarization multiplexer, thus reducing the existence of polarization-dependent gain in the Raman amplifier. However, the cost of using an additional laser may be a significant financial burden and, moreover, the combination of the two pumps may induce beating, which is particularly troublesome in a co-propagating Raman amplifier configuration.
Thus, a need remains in art for an improved pump arrangement for use with Raman amplified systems to minimize the effects of polarization-induced amplification penalties.
The need remaining in the prior art is addressed by the present invention, which relates to Raman amplified optical systems and, more particularly, to a Raman amplified WDM optical system using a polarization scrambled pump source to reduce the effects of polarization-dependent amplification penalties.
In accordance with the present invention, a pump input signal is subjected to polarization scrambling prior to being injected into the fiber amplifier to guarantee a random state of optical polarization for all input wavelengths within the amplifier. In this manner, the penalties or fluctuations due to the polarization state of the pump will be spread equally among all wavelengths and guarantee better stability.